<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        *{margin:0;padding: 0}
        .content{
            margin:100px;
            width:200px;
            height: 400px;
            background: #333;
            position: relative;
        }
        .content::before{
            content:"";
            display: block;
            width:0;
            height: 0;
            border:20px solid transparent;
            border-bottom-color:#333;
            position: absolute;
            /* z-index: 100; */
            top:-40px;
            left:50%;
            transform: translateX(-50%)
        }
    </style>
</head>
<body>
    <div class="content">

    </div>
</body>
</html>